MINOR: checks: Add the sni option for tcp-check connect rules

With this option, it is possible to specify the SNI to be used for SSL
conncection opened by a tcp-check connect rule.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 40585b8..8aaf912 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -9826,6 +9826,8 @@
 
     ssl          opens a ciphered connection
 
+    sni <sni>    specifies the SNI to use to do health checks over SSL.
+
     linger    cleanly close the connection instead of using a single RST.
 
     Examples: